Various clinching tools are known and usually comprise a punch operable in conjunction with a complementary die to plasticly deform the overlapping portions of metal and form a clinch which fastens the sheets together.
One such device includes a multi-segmented die bounded by a flexible restraining band permitting the die to resiliently open during the clinching operation. In the open configuration, however, particulate debris is permitted to migrate into the clearances between adjacent die segments. Repeated use causes the die to become clogged which prevents efficient operation of the tool.
Known clinching tools also suffer from an additional problem in that the wall thickness of the sheet material in the vicinity of the neck of the clinch tends to be significantly reduced because of the way in which the metal is extruded into the die. In some cases, the wall thickness in the region of the clinch can be reduced by in excess of 80% of the nominal gauge thickness of the metal, which significantly reduces the maximum shear strength of the clinch. In addition, the joint so formed is highly stressed in the vicinity of the clinch and therefore more susceptible to corrosion which directly affects the longevity of the joint. In many applications, for example in the building industry, these problems have prevented the widespread commercial acceptance of clinching as a viable means of assembly and construction in sheet metal because of the resultant difficulty encountered in meeting stringent safety requirements.
In addition to the problems of reduced wall thickness, inadequate shear strength, and reduced corrosion tolerance, the side walls of the die in so called "fixed die" tools must either be parallel or diverge outwardly in order to permit release of the clinch from the die. This inherent restriction in fixed die devices limits the maximum degree of interlocking mechanical engagement between the metal sheets forming the clinch and consequently limits the maximum "pull-out" strength of the joint.
It is therefore an object of the present invention to provide an improved clinching tool which overcomes or substantially ameliorates at least some of these disadvantages of the prior art.